Coatings used for the repair of a vehicle surface comprise several layers of different coating compositions such as primer, basecoat and clearcoat. Primer coating compositions are usually the initial coating layer applied in a vehicle coating repair process. The primer may be applied directly to bare metal or to an electrocoated substrate. In refinish applications a primer may also be applied to a substrate having one or more coating layers thereon. It is desirable that the coating cures quickly and that it provide a layer of paint that is sandable with sandpaper or other abrasive to remove imperfections from the coating layer. Typically the thickness of the coating layer is from 25 μm to 100 μm.
One class of coating compositions, well known for use in primers, comprises a hydroxyl functional polymer, such as polyester or acrylic polymer, and a polyisocyanate. These two components react together after application to the substrate to form a polyurethane coating. These compositions are often thus referred to as 2K polyurethane coating compositions, although technically the polyurethane is formed through crosslinking. 2K Polyurethane primers of this general type have been known for at least 20 years.
One problem with using such 2K polyurethane primers is that sufficient time must be allowed for curing to occur before it can be sanded and topcoated. Typically a primer must cure for 2-4 hours at ambient temperature. An additional drawback to polyurethane primers is that they contain relatively high levels of organic solvents. Due to environmental considerations there is a general trend in the coatings industry towards coating compositions with reduced solvent content.
Primer/sealant compositions curable under ultraviolet and thermal conditions are disclosed in PCT/EP00/03401. These coating compositions however require resins with ultraviolet curable groups and thermally curable functionality such as isocyanate functionality. Thermal cure must be applied in addition to the ultraviolet cure to achieve adequate cure of the coating composition, requiring additional energy expenditure and longer curing times.
Primer compositions curable under ultraviolet conditions only are disclosed in WO-01/74499 A1. This application discloses a primer coating and process for applying a primer coating to a substrate. This coating cures only under ultraviolet light.
The present invention provides a primer coating composition curable under natural outdoor light or by ultraviolet lamp radiation. The coating dries quickly, is sandable and provides adhesion to bare metal, as well as corrosion and humidity resistance.